I didn't do too well on the UMAT, but I can speak from experience that your preparation should get underway as soon as your Unit 3/4 exam is over( if you have any).
If you decide to leave preparation until the holidays before the exam, then your vce subjects may be compromised, as was the case with me.
In terms of approaching umat preparation:
I found a prep course to be quite useful. If you do choose one, then begin with the drills, or whatever other 'warm-up' activities they offer.
After you are comfortable with the layout of the exam, begin to do a few practice exams. You needn't do all three sections in one go, especially if you have a busy schedule. Expect to perform relatively poorly when you begin. With practice, your scores will increase naturally.
Identify which section is your weakest. Try to focus most of your efforts on this section. Ask yourself what it is about the section which gives you problems. Is it due to poor time management? Inability to comprehend the question? Reading too slowly?
When the issue has been identified, invest some time in trying to overcome it. For instance, if you find yourself working too slowly through the paper, then get into the habit of spending a maximum of 1.5 minutes on any given question. If you can't work it out in the allocated period of time, guess and move on. Force yourself to work under these conditions and soon, progress will be made.
One last tip: Anything can happen with the UMAT. Section 2 was consistently my best section in practice exams and drills, but turned out to be my worst on the day of the exam ( I ended up only scoring in the mid-50's). Section 3, on the other hand, went from worst to best. Expect anything and everything. After each section, regardless of how you thought you did, forget about it. Focus on the next section. Often, the people who score the highest get a score in the 50's for one section, and end up getting 75+ in another section.
Best of luck to you and the rest of the cohort for next year